Animal-derived collagen fiber tissue materials commonly used in the current clinical practice, such as bioprosthetic biovalve, transcatheter aortic valves and biological patches like bovine pericardium, small intestinal submucosa, etc., are usually preserved with chemical reagents such as glutaraldehyde and/or formaldehyde, or treated with such chemical reagents for crosslinking. And tissues are placed in dilute aqueous solutions containing glutaraldehyde and/or formaldehyde so that the components of the tissues are kept in a sterile environment and hence maintain their hydrated state typically. However, extensive studies have shown that glutaraldehyde remaining in implanted prostheticbiovalve can promote the calcification of the biovalve. Absence of aldehydes during the preservation can lead to significantly slower calcification of the biovalve. Mirzaie et al. preserved a porcine bioprosthetic aortic valve in a solution based on human serum solution in lieu of a glutaraldehyde solution and confirmed a reduction of about 50% of calcium in the valve. In addition, glutaraldehyde has a high toxicity, and even a very small glutaraldehyde residue may have a poisoning effect on the human body and affect the formation of endothelium (more details in this regard can be found in Mirzaie M, Brunner E, Mahbub-ul Latif A H, et al. A new storage solution for porcine aortic valves. Ann Thorac Cardiovasc Surg, 2007, 13:102-109). So, bioprosthetic tissues treated or preserved with glutaraldehyde must be rinsed many times in order to remove glutaraldehyde before the implantation. Further, as exposure to the aldehydes contained in such preservative solutions may cause a significant harm to health, additional protective measures are necessary for the production workers, medical personnel and patients, leading to increased costs and inconvenience. It is conceivable that reducing medical staff's preparatory efforts as much as possible prior to the implantation of biological tissue materials and prostheses prepared therefrom by providing them in a form ready for use can reduce not only the chance for infections or faults but also the time required for the implantation. Thus, development of novel methods for dry state preservation of biological tissues is promised with good prospects and is of high application value.
Presently, biological tissues are often preserved in a lyophilized form by converting water in the tissues into ice crystals at −80° C. and then freeze-drying them at reduced pressures (vacuum drying) to obtain the dry tissues. However, such lyophilized biological tissues are not tough and prone to breaks or fractures due to a lack of moisture and hydrophilic solvents, and their production cost is relatively high. Moreover, the formation of ice crystals may lead to structural damage to the tissues, and it is difficult for the dry biological tissues to be rehydrated and it takes a few days to recover their original hydrated state.
Chinese Patent Pub. No. CN1306445A describes a method for treating a tissue with a solvent. The method involves treating the biological tissue with an increasing gradient of concentration of a polar organic solvent (selected from methanol, ethanol, isopropanol, acetonitrile, acetone and butanone) and then treating the biological tissue with a solution containing glycerol or low molecular weight (<1000 D) polyethylene glycol, and polyethylene glycol of a molecular weight between 6,000 D to 15,000 D and heparin. After that, the biological tissue is immersed in an aqueous heparin solution shortly, frozen and lyophilized. However, this dehydration process significantly reduces the overall size of the tissue and the chemical reagents used (e.g., acetonitrile, acetone, etc.) is toxic. Additionally, the biological tissue obtained from this dehydration process cannot be rehydrated and recovered to its original size.
Fumoto et al. reported absence of significant deterioration in size, shape or in vitro pulsed flow performance of a tissue after it was subjected to treatment with a 57% aqueous glycerol solution, drying in an environment with a relative humidity of less than 28% for 6 to 8 hours and sterilization with ethylene oxide (EO). Reference can be made to Fumoto H, Chen J F, Zhou Q, et al. Performance of bioprosthetic valves after glycerol dehydration, ethylene oxide sterilization, and rehydration. Innovations (Phila), 2011, 6(1): 32-36 for more details in this regard.
Chinese Patent Pub. No. CN101965205A describes a method for preparing a biological tissue or prosthesis by dehydration and drying with a glycerol/ethanol (75%/25%) mixture.
Chinese Patent Pub. No. CN103933612A relates to a biological tissue for surgical implantation. The biological tissue is treated with a non-aqueous solution including a polyhydric alcohol (selected from glycerol, propylene glycol, glycerol derivatives, propylene glycol derivatives and mixtures thereof) and a C1-C3 alcohol (selected from methanol, ethanol, isopropanol and n-propanol), and a portion of the solution is removed from the solution-treated biological tissue. This method can maintain the tissue in a substantially dry state. However, the treated tissue tends to bend or warp subsequent to the partial removal of the treatment solution and contains residues of the treating agents which are hard to be removed and may affect the sterilization process. In addition, treatment simply with the alcohols fails to allow effective control of the remaining water content of the treated tissue in the “substantially dry” (a water content greater than 30% will affect the sterilization performance of EO and lead to insufficient sterilization).
To sum up, the existing biological tissue preservation methods, such as those using aldehyde solutions or lyophilization as mentioned above, have a number of disadvantages such as toxicity of the residual reagents, morphological deformation of the tissue and high cost and thus need modifications and improvements in terms of application. While the glycerol-based non-hydrophilic dehydration methods to dry animal-derived collagen fiber tissue are relatively advantageous, they still need further improvements in morphological deformation of the biological tissue, residues of the reagents and water content variations.